Carbon–phosphorus bond formation and transformation in the reaction of 1,2-diaza-1,3-butadienes with alkyl phenylphosphonites Orazio A. Attanasi a, * , Graziano Baccolini b , Carla Boga b , Lucia De Crescentini a, * , Paolino Filippone a , Fabio Mantellini a a Istituto di Chimica Organica, Universita ` degli Studi di Urbino ‘Carlo Bo’, Via I Maggetti 24, 61029 Urbino, Italy b Dipartimento di Chimica Organica ‘A. Mangini’, Universita ` degli Studi di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy article info Article history: Received 18 February 2008 Received in revised form 10 April 2008 Accepted 1 May 2008 Available online 3 May 2008 Keywords: 1,2-Diaza-1,3-butadienes Diazaphospholes Ylides Hydrazones Phospha-Michael addition abstract The reaction of 1,2-diaza-1,3-butadienes with dialkyl phenylphosphonites under solvent-free conditions proceeds via zwitterionic intermediate and gives, by precipitation, the stable ylidic a-phosphanylidene- hydrazones that, in turn, can be transformed into the corresponding 3-phenyl-2H-1,2,3l 5 -diazaphosp- holes. The latter compounds are converted by hydrolytic cleavage in methanol–water (95:5) into E-hydrazonophosphonates that are useful for the preparation of the corresponding b-ketophosphonates and 4-[alkoxy(phenyl)phosphoryl]-1,2-diaza-1,3-butadienes. These peculiar 1,2-diaza-1,3-butadienes, bearing an alkoxy(phenyl)phosphoryl group on the carbon atom in position 4 are also able to add dif- ferent nucleophiles, such as methanol or thiourea, giving 2-[alkoxy(phenyl)phosphoryl]-2-methoxyhy- drazones and 5-phosphinate-substituted thiazol-4-ones, respectively. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Since the 1960s, investigations regarding 1,2-diaza-1,3-butadi- enes have drastically increased, showing the usefulness of these compounds in organic chemistry. 1,2 Our group has a 30-year ex- perience in this field. In particular, we have studied the reactivity of these substrates, both in solution and solid phase, 3 toward a large variety of nucleophiles. 1 The 1,4-conjugate addition (Michael-type) of the latter on the terminal carbon atom of the heterodiene system and the subsequent internal ring closure of the 1,4-adduct hydra- zonic intermediates form polyfunctionalized five- or six-membered heterocycles, such as pyrroles, 3a,4 thiazoles, 5 thiazolinones, 3a,c pyrazoles, 3a,6 thiadiazoles, 7 selenadiazoles, 7 indoles, 8,4b,c imidaz- oles, 9 1,2,4-triazines, 1c pyrazines, 10 and pyridazines. 11 On the other hand, the phospha-Michael (P-Michael) addition, i.e., the addition of a phosphorus nucleophile to an appropriate acceptor, is probably one of the most versatile tools for the for- mation of the P–C bond. 12 This occurrence is important because it offers an entry to many diversely functionalised derivatives. In addition, natural products containing a P–C bond exhibit important biological activity. 13 In the past years, all these considerations suggested us to examine the reaction between 1,2-diaza-1,3-butadienes and tri- phenyl- 14a or trialkyl-phosphines 14b to obtain pyrazoles or 4- phosphoranylidene-4,5-dihydropyrazol-5-ones. More recently, we have investigated the reaction of the same 1,2-diaza-1,3-butadienes with trialkyl phosphites that represented a facile access to new 1,2,3-diazaphospholes, if the reaction was carried out under a ni- trogen atmosphere, or to E-hydrazonophosphonates, in the pres- ence of atmospheric moisture. 15 With the aim to study the behavior of other trivalent phosphorus nucleophiles towards 1,2-diaza-1,3-butadiene systems and consid- ering the different electron-donating effects of the phenyl group with respect to the alkoxy one, in this paper we consider the reaction of these compounds with dimethyl or diethyl phenylphosphonites. It turned out to be very interesting because, in some cases, the 1,4- addition products directly precipitate from the reaction medium, permitting us to define the mechanistic aspects of the reaction. On the other hand, with the same reaction pathway, we can obtain 3-phenyl-2H-1,2,3l 5 -diazaphospholes, also in the presence of air. Furthermore, the procedure here described represents an easy alternative route to the synthesis of 1,2,3-diazaphospholes. In fact, the classical method for their preparation implicates the PCl 3 condensation of the alkylketone hydrazones 16 while 2,4,5-tri- phenyl-1,2,3-diazaphosphole is best prepared from the respective 1,2-diaza-1,3-butadiene and a fused benzothiadiphosphole as the phosphorus donating reagent. 17 * Corresponding authors. Fax: þ39 0722303441. E-mail addresses: orazio.attanasi@uniurb.it (O.A. Attanasi), lucia.decrescentini@ uniurb.it (L. De Crescentini). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2008.05.007 Tetrahedron 64 (2008) 6724–6732